1. Field of the Invention
The invention relates to a thickening agent which modifies the rheological characteristics of charged and/or pigmented, white or colored aqueous compositions.
2. Discussion of the backgrounds
For one of skill in this art, a charged and/or pigmented aqueous composition is formed from a liquid phase which can be water or a water-miscible organic solvent, or even a mixture of both, from a polymer in emulsion in the liquid phase, which is called a "binder", from charges and/or pigments, from an agent dispersing the charges and/or pigments which can be a water-soluble polymer or copolymer, from additives as varied as coalescence agents, biocidal agents, antifoaming agents or others, and finally from a thickening agent which is a natural or synthetic polymer or copolymer.
For a long time it has been shown that the presence of a thickening agent in a charged and/or pigmented aqueous composition was necessary in order to modify the rheological characteristics thereof.
Several species of thickening agents for charged and/or pigmented aqueous compositions have been proposed to the skilled artisan and have been described abundantly in the specialized literature.
A first species of thickening agents for charged and/or pigmented aqueous compositions consists of cellulose derivatives with the property of viscosifying only the aqueous phase of said compositions.
However, the field of application of this first species of thickening agents is limited, because disadvantages occur, causing problems to the user such as, for example, difficulties in dissolving in aqueous media, frequently slow hydration kinetics, high bacterial sensitivity, and also an inability to cause to develop, and more specifically to adjust afterwards, the viscosity of the charged and/or pigmented aqueous compositions.
Thus, it is apparent that this first species of thickening agents causes, within the charged and/or pigmented aqueous compositions in which it is used, pseudo-plastic rheological characteristics which can be troublesome, such as a high viscosity in the absence of mechanical stress which, however, decreases very considerably under the effect of shearing, resulting in poor covering power.
Another species of thickening agents has been proposed to overcome certain above-identified disadvantages of this first species. This other species is formed from synthetic latexes, which may be acrylic, illustrative descriptions of which are given for example in patents such as FR 2,131,128, FR 2,281,389, U.S. Pat. No. 2,798,053, U.S. Pat. No. 2,985,625 or even U.S. Pat. No. 2,958,679.
Thus, FR 2,281,389 describes a cross-linked polymer of maleic anhydride and ethylene in the presence of trialkyl isocyanurate, which can be used as a thickening agent for aqueous systems such as synthetic latex-based paints, mineral flocculating treatments, industrial or domestic water coagulation treatments.
In general, these thickening agents are carboxylic acid polymers with an ethylene function or copolymers of said same acids and their esters presented in the form of a low viscosity aqueous emulsion, of the oil in water type.
Soluble in aqueous and alkaline media, said thickening agents have the advantage, in relation to the above-identified cellulose derivatives, of being more easily used and of being insensitive to bacterial attacks.
However, as with the first species of thickening agents, the second species has certain disadvantages, such as causing the thickening of only the aqueous phase of said compositions, or even of conferring on said compositions rheological characteristics which are too pseudo-plastic and which are not suitable for all paint formulations.
More recently, a last species of water-soluble thickening agents has appeared in the field of charged and/or pigmented aqueous compositions. These thickening agents are traditionally called associative thickening agents, because they act not only by increasing the viscosity of the aqueous phase (by solubilization), but also by creating various bonds between the copolymer and certain components of the compositions, very probably through the appearance of hydrophobic interactions and hydrogen bonds. The advantage of these associative thickening agents, in comparison with the above-identified agents, is to provide the charged and/or pigmented compositions in which they are used with a less pseudo-plastic behavior.
In particular, in the case of certain paints, the skilled artisan seeks to obtain a viscosity under low shearing which is fairly low so that the film deposited on the support to be protected has a tendency to level out well the irregularities in thickness due to the application (appropriate film tension), and has a viscosity under high shearing which is sufficiently high to improve the covering power and to decrease projections when said paints are applied using a roller.
This last species of associative thickening agents has been developed into two families, that of polyurethane associative thickening agents and that of acrylic associative thickening agents.
The polyurethane associative thickening agents belonging to the first family contain in their molecule one or several polyether chains terminating with hydrophobic groups, such as, for example, alkyls, aryls or alkylaryls, and are obtained by condensation chemistry.
Such agents are described in numerous patents, for example in GB 1,069,735, U.S. Pat. No. 3,770,684, U.S. Pat. No. 4,079,028 and U.S. Pat. No. 4,155,892.
However, while these agents confer on the charged and/or pigmented compositions in which they are used desirable rheological characteristics, they cause certain troublesome disadvantages for their user.
In effect these agents are in a viscous form which is not easy to handle, possibly in solution in mixtures of water and solvent(s), with the solvent possibly being more or less toxic and, thus, subject to limited use, or even has reactive incompatibility with certain components of the charged and/or pigmented compositions.
The acrylic associative thickening agents obtained by radical polymerization, belonging to the second family, which are water-soluble in a neutral or alkaline medium, are formed from copolymers in general prepared from ethylenic carboxylic acids, possibly esters of said acids and/or other monomers, and finally from at least one particular functional monomer having a lateral chain composed of polyether groups containing hydrophobic, hydrocarbon terminal radicals.
The nature of the particular functional monomer has been shown to be a determining factor in the rheological action of the charged and/or pigmented compositions containing the corresponding thickening agent.
Thus, the particular functional monomer can be a surfactant alcohol acrylate or methacrylate (EP Patent 0,013,836 and U.S. Pat. No. 4,384,096), or can result from esterification with a surfactant alcohol of acrylic acid oligomers (U.S. Pat. No. 4,421,902). This particular functional monomer can also be an oxyethyl ester of crotonic acid (U.S. Pat. No. 4,569,965), or also a maleic anhydride hemiester (EP Patent 0,248,612), or even a surfactant ether of allyl alcohol (EP Patent 0,216,479).
This particular functional monomer can finally result from the condensation of a surfactant alcohol and an unsaturated isocyanate (US Pat. No. 4,514,552 and U.S. Pat. No. 4,600,761), with the presence of --O--C(0)--NH--urethane groups on the lateral chains of the copolymer providing a beneficial effect on the rheological behavior of the charged and/or pigmented compositions and in particular aqueous paints.
The object sought by the use of acrylic associative thickening agents was to modify, in the most favorable manner, the rheological characteristics of said compositions through their presence, such that they preferably have a controlled viscosity simultaneously under high and low shearing, so as to have good covering power, acceptable sag resistance and an appropriate film tension.
However, it has been noted that, among the acrylic associative thickening agents of the prior art:
certain ones already had an acceptable rheological profile, but require of large quantities to be used which the formulator desired to see decreased,
the others, such as those indicated in U.S. Pat. No. 4,514,552, did not manage to control simultaneously the viscosities under high and low shearing.